Many processes are known for formation of color images by transfer or diffusible dyes. For example, U.S. Pat. No. 3,134,764 discloses a process using a dye developing agent in which a hydroquinone developing agent is bonded with a dye. Japanese Patent Application (OPI) No. 63618/76 discloses a process which comprises releasing a diffusible dye by an intramolecular ester interchange reaction, and Japanese Patent Application (OPI) No. 111628/74 discloses a process which comprises releasing a diffusible dye by an intramolecular rearrangement reaction of a isooxazolone ring. All of these processes are those wherein the dye is diffused into an image receiving layer in parts where development does not take place (nonexposed parts), and release or diffusion of the dye does not take place in parts where development is carried out (exposed parts). However, these processes have a fault in that it is very difficult to obtain an image having a high ratio of S/N (signal/noise), because the development is carried out simultaneously with the release or diffusion of the dye. In order to overcome this fault, a process wherein a coloring material which is provided in an oxidized state having no ability to release a dye is allowed to coexist with a reducing agent precursor, and a diffusible dye is released by reduction with a reducing agent remaining without oxidation after development, is described in Japanese Patent Application (OPI) No. 110827/78. However, this process has a fault in that an image having a high ratio of S/N is not obtained if the development rate and the reduction rate of the coloring material by the reducing agent are not strictly controlled.
On the other hand, as a process wherein a diffusible dye is released in the part where development is carried out, a process which comprises releasing a diffusible dye by a reaction between a coupler having a diffusible dye in a releasing group and an oxidized product of a color developing agent is described in British Pat. No. 1,330,524, and a process which comprises forming a diffusible dye by a reaction between a coupler having an antidiffusible group in the releasing group and an oxidized product of a color developing agent is described in U.S. Pat. No. 3,227,550. However, in these processes using a color developing agent, staining of the image by the oxidized product of the developing agent causes serious trouble.
As reducing dye-providing substances which have a similar function with compounds of the present invention, the following compounds (1) to (12), etc., have been known. ##STR1##
Of these, the compounds (1) to (9) release a diffusible dye having a terminal sulfamoyl group by the function of alkali after they are oxidized by development processing. However, they require an alkali solution having a high pH as a developing solution and have a fault in that it is necessary to use a hydrophilic binder and to use an alkali solution as a diffusing assistant in order to obtain an image having high density, because the released dye has very low diffusibility in a hydrophobic binder because of the sulfamoyl group.
Further, the compounds (10) to (12) cannot be put to practical use, because efficiency in the dye releasing step after oxidation is inferior.